1. Field of the Application
The present invention relates to metallurgy, and more particularly, to a method for the production of two-layer pipes.
The products of this invention can be advantageously used in the chemical industry in the manufacture and shipment of acids and alkalies under conditions of high temperature and pressure, in the transportation of molten metals, such as aluminum melt, for long distances, as well as in the transportation of abrasive dispersion media, such as SiO.sub.2, SiC, etc.
At present, the production of pipe casting is rather complicated procedure which includes the stages of melting the materials required for different layers in a melting furnace, pouring the resultant melts into a centrifugal apparatus, and centrifugal forming of an internal layer and then of an external layer; it also involves substantial expences required for the melting equipment and electric power.
In a number of cases a finished pipe is used as the external layer of a two-layer pipe. The production of this type of pipe is likewise complicated in that it includes the stages of melting the material for the pipe internal layer in a melting furnace, pouring the resultant melt into a rotational tube, and effecting centrifugal formation of the internal layer; the production process also involves great expense for the melting equipment, and requires considerable consumption of electric power and of an initial material.
It is therefore expedient to render the production of two-layer pipes more simple and economical.
2. Description of the Prior Art
For example, there is known a method for producing two-layer pipes in a centrifugal apparatus (cf. L. A. Appen, High-Temperature-Resistant Inorganic Coatings, "Chemistry", 1967, /in Russian/), which consists in that a molten metal, such as stainless steel, is poured into a rotating metal tube and is then spread under the action of centrifugal forces over the surface of the metal tube to thereby closely adhere thereto. The production of pipes according to the above-described method is both inefficient and unprofitable, since it requires great expenditures for the melting equipment and initial material, it also involves additional expense for the transportation of the melt to centrifugal apparatus, as well as for electric power.
There is also known a method for producing two-layer pipes (cf. USSR Inventor's Certificate No. 97637, cl. B22 13/02), which comprises the stages of pouring molten steel into a centrifugal apparatus, cooling the melt formed as a cylinder-shaped layer to a temperature of 1320.degree. to 1350.degree. C., subsequent pouring of ferrosilicide or of stainless steel onto the first layer, and cooling down the finished two-layer pipe.
The production of pipes in accordance with the method described in the patent referred to above is complicated and unprofitable. As in previous cases, it involves appreciable expense for the costly melting equipment and initial material, as well as for the electric power input and transportation of the melt to a centrifugal apparatus.
U.S. Pat. No. 4,005,741 discloses a method for the production of multilayer pipes, which comprises successive introduction into a rotational tube of a powdery heat-insulating material, such as Al.sub.2 O.sub.3 or SiO.sub.2, and then of a powdery mixture of aluminum with iron oxide (thermit mixture) in a weight ratio of the heat-insulating material to thermit mixture of 1:10; the thermit mixture is subsequently ignited by means of electric-arc welding with the droplets of the melted-down electrode. Centrifugal acceleration is kept within the range of 83 to 89 g and is thereafter maintained constant until separation of the iron melt and aluminum oxide into two layers, followed by their solidification. The operating process is effected under atmospheric pressure.
The pipe produced according to the method described above is nonuniform in thickness due to uneven distribution of the initial mixture over the surface of the rotational tube wherein said mixture is introduced. In addition, the delivery of the initial mixture to the rotational tube requires a special flow-metering device which adds to the cost of the production process.
The internal layer of the pipe, produced in accordance with the above-described method and composed of Al.sub.2 O.sub.3, tends to be porous because of vigorous evaporation of aluminum, occurring during its combustion when effected under atmospheric conditions, as well as by reason of insufficiently high centrifugal acceleration enabling the removal of gaseous aluminum.
The intermediate layer of the pipe, composed of iron resulting from the reaction, is contaminated with oxygen contained in the atmosphere and entering into reaction with the molten iron, which leads to the brittleness of the multilayer pipe.
The method of the patent referred to above makes it impossible to produce multilayer pipes from the mixture of oxides of transition metals with a reducing agent, such as Al or Mg, the reaction heat of which, and, consequently, the combustion temperature, are very high (3500.degree. to 4500.degree. C.); under atmospheric conditions the combustion is fraught with explosions and expulsion of the reaction mass.
In addition, the above method fails to provide for good bonding between the pipe layers due to the absence of the Van der Waals force between the metals forming the pipe intermediate layer and the oxides forming the pipe internal layer.
This method makes use of finished pipes as the initial material which, needless to say, is rather costly.
According to another embodiment of the above-described method, the production of two-layer pipes is carried out without resorting to the use of finished metal tubes. In accordance with this embodiment, a reaction chamber is filled with an initial mixture composed of iron and aluminum oxides. The mixture is then inflamed after being set in rotation about the longitudinal axis of the reaction chamber. Under the action of centrifugal force the resultant melt is separated into two layers, external and internal, due to the difference in specific weights of the final products of combustion. The initial mixture is inflamed at a rotating speed of 83 to 98 g, which is maintained constant until crystallization of the melt layers.
The pipe made according to the said method is nonuniform in thickness due to the uneven distribution of the initial mixture in the process of introducing it to the rotational tube. Furthermore, the delivery of the initial mixture to the rotational tube requires a special flow-metering device which increases the cost of the production process.
The internal layer of the pipe produced in accordance with the above-described method and composed of Al.sub.2 O.sub.3 tends to be porous due to the intensive evaporation of aluminum during combustion under atmospheric conditions, and also due to insufficiently high centrifugal acceleration facilitating the removal of gaseous aluminum.
The external layer of the pipe consisting of iron formed during the combustion of the initial mixture, is contaminated with oxygen which is present in the atmosphere and reacts with the melted iron, as a result of which a brittle multilayer pipe is produced.
The method does not make it possible to obtain two-layer pipes from a mixture of oxides of transition metals with a reducing agent, such as Al or Mg, the reaction heat of which and consequently, the combustion temperature, are very high (3500.degree.-4500.degree. C.), and under atmospheric conditions, the combustion is fraught with explosions and expulsion of the reaction mass.
The method does not ensure a durable bonding between the pipe layers due to the absence of the Van der Waals force between the metal layer and the oxide layer.
It is therefore an object of the present invention to enhance efficiency and simplify the production process of a method for producing two-layer pipe casting.
Another object of the invention is to improve quality of two-layer pipe casting by removing porosity in the pipe internal layer, enabling stronger bonding between the internal and external layers of the pipe casting and ensuring uniform thickness of the two layers.
Still another object of the invention is to enhance chemical resistance of two-layer pipe casting to corrosive media at a temperature of 1500.degree. to 2500.degree. C.